The present invention relates to a deflection yoke attached to a color cathode ray tube forming multiple electron beams arranged in line, and in particular to a deflection yoke having convergence correction means.
A conventional deflection yoke having convergence correction means is described in "Low leakage magnetic field SS deflection yoke for 14" color dispaly CRT", Technical Report of The Institute of Television Engineers of Japan, Vol. 16, No. 2 (January 1992), IDY92-18, pp. 31-35, for example. In parallel with vertical deflection coils formed by two coils, resistors and a variable resistor are connected. Balance of a magnetic field generated by the vertical deflection coils formed by the two coils is changed by the variable resistor. Misconvergence of horizontal lines formed at upper and lower ends of a screen by electron beams located at opposite sides of multiple electron beams is corrected simultaneously in upper and lower parts of the screen.
FIG. 1 is a circuit diagram showing connection relations in such a conventional deflection yoke having convergence correction means. In FIG. 1, vertical deflection coils 4a and 4b formed by two coils are connected in series. Across the two vertical deflection coils, a series combination of resistors 10a and 10b and a variable resistor 11 is connected in parallel therewith. Furthermore, a variable terminal of the variable resistor 11 is connected to a junction between the vertical deflection coils 4a and 4b.
In the configuration heretofore described, an unbalance can be caused in vertical deflection currents flowing through the vertical deflection coils 4a and 4b by changing values of resistors respectively connected in parallel with the vertical deflection coils 4a and 4b by means of the variable resistors 11.
FIGS. 2A, 2B, 3A and 3B are sectional views showing sections of vertical deflection coils in the conventional deflection yoke illustrated in FIG. 1. In FIGS. 2A, 2B, 3A and 3B, numeral 13 denotes a vertical deflection magnetic field, 14a and 14b senses of vertical deflection currents, 15a and 15b senses of changes of vertical deflection currents, 16 a change of a vertical deflection magnetic field, 17B, 17G and 17R electron beams, and 18B and 18R deflection forces applied to the electron beams 17B and 17R.
FIG. 2A shows senses 14a and 14b respectively of vertical deflection currents let flow through the vertical deflection coils 4a and 4b when the electron beams 17B, 17G and 17R are deflected to the upper part of the screen. FIG. 2A also shows the vertical deflection magnetic field 13 generated at that time. In the same way, FIG. 2B shows senses 14a and 14b respectively of vertical deflection currents let flow when the electron beams 17B, 17G and 17R are deflected to the lower part of the screen. FIG. 2B also shows the vertical deflection magnetic field 13 generated at that time. In FIGS. 2A and 2B, it is assumed that the position of the variable terminal of the variable resitor 11 shown in FIG. 1 is at the middle point thereof.
On the other hand, FIGS. 3A and 3B show senses 15a and 15b of changes of vertical deflection currents and the change 16 of the vertical deflection magnetic field when the position of the variable terminal of the variable resistor 11 is moved from the states of FIGS. 2A and 2B toward the resistor 10a. That is to say, FIG. 3A shows them obtained when the electron beams 17B, 17G and 17R are deflected to the upper part of the screen, whereas FIG. 3B shows them obtained when the electron beams 17B, 17G and 17R are deflected to the lower part of the screen.
When the electron beams 17B, 17G and 17R are deflected to the upper part of the screen, all senses are reversed as compared with when the electron beams 17B, 17G and 17R are deflected to the lower part of the screen.
With reference to FIG. 1, the value of the resistor connected in parallel with the vertical deflection coil 4a is decreased by moving the position of the variable terminal of the variable resistor 11 toward the resistor 10a. Thereby, the amount of the vertical deflection current diverted to the resistor 10a is increased and the value of the current flowing through the vertical deflection coil 4a is decreased. As shown in FIG. 3A (or FIG. 3B), therefore, the sense 15a of the change of the vertical deflection current is opposite to the sense 14a of the vertical deflection current shown in FIG. 2A (or FIG. 2B).
On the other hand, the value of the resistor connected in parallel with the vertical deflection coil 4b is conversely increased. Thereby, the amount of the vertical deflection current diverted to the resistor 10b is decreased and the value of the current flowing through the vertical deflection coil 4b is increased. As shown in FIG. 3A (or FIG. 3B), therefore, the sense 15b of the change of the vertical deflection current is identical with the sense 14b of the vertical deflection current shown in FIG. 2A (or FIG. 2B).
Since the senses 15a and 15b of changes of vertical deflection currents are such senses, the change 16 of the vertical deflection magnetic field has four-pole magnetic field components as shown in FIG. 3A or 3B. As shown in FIG. 3A or 3B, therefore, deflection forces 18B and 18R are applied to the electron beams 17B and 17R located at opposite sides. Paying attention to the blue electron beam 17B, the deflection force 18B is applied to the blue electron beam 17B downward as shown in FIG. 3A when the electron beams 17B, 17G and 17R are deflected to the upper part of the screen. When the electron beams 17B, 17G and 17R are deflected to the lower part of the screen, the deflection force 18B is applied to the blue electron beam 17B upward as shown in FIG. 3B.
The case where the position of the variable terminal of the variable resistor 11 shown in FIG. 1 is moved toward the resistor 10a has heretofore been described. Moving the position of the variable terminal of the variable resistor 11 toward the resistor 10b causes movements wholly opposite to those in the foregoing description.
FIGS. 4A and 4B are diagrams illustrating changes of a convergence pattern on a phosphor screen 19. FIG. 4A shows the case where the position of the variable terminal of the variable resistor 11 shown in FIG. 1 is moved toward the resistor 10a. FIG. 4B shows the case where the position of the variable terminal of the variable resistor 11 is moved toward the resistor 10b.
It is now assumed that the position of the variable terminal of the variable resistor 11 is located at the middle point prior to movement and a blue horizontal line 20B coincides with a red horizontal line 20R on the phosphor screen 19 at that time. If the position of the variable terminal of the variable resistor 11 is moved toward the resistor 10a, the blue horizontal line 20B moves toward the inside of the screen as shown in FIG. 4A. If on the contrary the position of the variable terminal of the variable resistor 11 is moved toward the resistor 10b, the blue horizontal line moves toward the outside of the screen.
In the conventional deflection yoke shown in FIG. 1, horizontal line misconvergence at upper and lower ends of the screen is corrected simultaneously in the upper and lower parts of the screen by thus moving a blue horizontal line 20B relative to a red horizontal line 20R simultaneously toward the inside simultaneously in both upper and lower parts of the screen or toward the outside simultaneously in both upper and lower parts of the screen.
In the conventional deflection yoke having convergence correction means, horizontal line misconvergence at the upper and lower ends of the screen is corrected simultaneously in the upper and lower parts of the screen by moving a blue horizontal line relative to a red horizontal line toward the inside simultaneously in both upper and lower parts of the screen or toward the outside simultaneously in both upper and lower parts of the screen as described above.
It is now assumed that horizontal line misconvergence as shown in FIGS. 5A and 5B has occurred as horizontal line misconvergence caused at the upper and lower ends of the screen by the electron beams located at opposite sides. However, all the above described conventional deflection yoke can do is to move blue horizontal lines inside relative to red horizontal lines simultaneously in both upper and lower parts of the screen or outside simultaneously in both upper and lower ends of the screen. If it is attempted to correct the horizontal line misconvergence at one of the upper and lower ends of the screen, therefore, the horizontal line is significantly displaced into a direction opposite to the correction direction at the other of the upper and lower ends of the screen. The conventional deflection yoke has such a problem.
With reference to FIG. 5A, a blue horizontal line 20B located in the upper part is moved toward the outside of the screen in order to make the blue horizontal line 20B coincide with a red horizontal line 20R. In this case, blue horizontal lines 20B move toward the outside in both upper and lower parts of the screen. In the upper part of the screen, therefore, the blue horizontal line 20B can be made coincident with the red horizontal line 20R. However, the blue horizontal line 20B located in the lower part of the screen moves in a direction opposite to the direction of coincidence and becomes farther away from the red horizontal line 20R.
In case of horizontal line misconvergence as shown in FIGS. 5A and 5B, therefore, adjustments are made for the conventional deflection yoke in such a state that horizontal lines are deviated by nearly the same distance in the upper and lower parts of the screen. Thus, misconvergence cannot be completely corrected.